Emissions from digestate – quantification and mitigation

Steve Bungay – Technical Director, Helix ECL
Matt Smyth – Technical Director, Aqua Enviro
David Tompkins – Head of Knowledge Exchange and Innovation, Aqua Enviro



Anaerobic digestion (AD) is widely used as a method for producing renewable energy via the biological stabilisation of sewage sludge and food waste slurries. During stabilisation, organic matter is broken down and converted to biogas, with the biogas comprising of mainly methane (CH4) and carbon dioxide (CO2).

To optimise the production of biogas, pre-treatment processes have been added upstream of AD, the organic loading to the AD plants has been increased, and hydraulic retention times (HRT) of both the primary digesters and post-digestion storage tanks have been reduced. Although the production of renewable energy from biogas has increased significantly, optimising biogas production without changing the fundamental design of AD plants will result in unforeseen increases in uncontrolled CH4 production and release, which not only a
greenhouse gas, but it will also increase the risk of explosion. To manage this risk, it is critical to understand the overall CH4 generation across the whole system. A simple method using first order kinetics was developed, which quantifies the CH4 emissions from digestate. The result of this modelling demonstrates that current practice does not provide adequate retention for reliable cessation of methanogenesis to occur.

With CH4 released after primary digestion, depending on the configuration of the biogas system, there is a risk of explosion at post digestion storage tanks, or at the digestate dewatering stage. To avoid this, there needs to be an integrated approach amongst achieving the desired biogas production, meeting the requirements for environmental regulations, without compromising the Dangerous Substances and Explosives Atmosphere Regulations (DSEAR) [1]. Understanding the digester operating parameters; the CH4 potential, the digester
stability, and the digester active capacity, are important components of improving the performance and safety of AD.

Key Words
Biogas, cessation, CH4, Clean Air Strategy, DSEAR, first-order kinetics, lower explosion limit, methane, methanogenesis, Net Zero, PDST.

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